The mobility of an unmanned vehicle is extremely dependent upon the terrain, obstacles in the path of the vehicle causing various mobility failures such as overturning, gross slippage, nose-in failure and hang-up failure, the latter two being caused by engagement of the nose and belly respectively of the vehicle with the terrain.
Susceptibility to such failures can be reduced to some extent by the use of large vehicle size as obstacles then become less significant but, for certain mobility objectives, eg a vehicle that could go anywhere a man could walk, excessive dimensions are not practicable. Walking machines have been often proposed for high mobility, and it is certainly true that many animals with legs are highly mobile and if one is attempting to duplicate their capabilities it is reasonable to consider using similar mechanisms. However, the potential for mobility failure is much higher than that of more conventional vehicles, as each foot must be safely placed and moved in such a way as to produce the desired vehicle motion. Simple control strategies can be used but these do not in general produce the mobility apparent in legged animals. For these reasons fully fledged walking machines are not practical at present although some mobility aids are known which go some way in this direction.
Wheeled or tracked vehicles are more appropriate for unmanned use, but the width necessary for stable operation of most conventional vehicles prevents them from following many routes that are easy for a man. A motorcycle has very good mobility from many respects but is vulnerable to a great many failure modes and is only useful when allied with a competent operator. Very small tracked vehicles are also known which are designed to be able to operate inside buildings and to climb stairways, but these are unable to carry a large payload and are defeated by fairly simple obstacles.
Wide track vehicles of larger size are known for coping more effectively with obstacles, the substantially full underside cover provided by the twin wide tracks used making the vehicle bellyless and hence less susceptible to hang-up failure. However these vehicles need substantial width for effective steering, the mode of steering normally employed being skid steering, in which the two independently drivable, parallel tracks are made to skid about a pivot axis trajectory determined by the velocity differential of the two parallel tracks and by the load distribution of the vehicle upon them.
The present invention seeks to provide a bellyless vehicle of narrower aspect capable of an alternative steering mode.